The present invention relates to processing omnidirectional video content, and more particularly, to a de-blocking method for a reconstructed projection-based frame that employs a projection layout of a 360-degree virtual reality (360 VR) projection.
Virtual reality (VR) with head-mounted displays (HMDs) is associated with a variety of applications. The ability to show wide field of view content to a user can be used to provide immersive visual experiences. A real-world environment has to be captured in all directions resulting in an omnidirectional image content corresponding to a sphere. With advances in camera rigs and HMDs, the delivery of VR content may soon become the bottleneck due to the high bitrate required for representing such a 360-degree image content. When the resolution of the omnidirectional video is 4K or higher, data compression/encoding is critical to bitrate reduction.
Data compression/encoding of the omnidirectional video may be achieved by a conventional video coding standard that generally adopt a block-based coding technique to exploit spatial and temporal redundancy. For example, the basic approach is to divide a source frame into a plurality of blocks (or coding units), perform intra prediction/inter prediction on each block, transform residues of each block, and perform quantization and entropy encoding. Besides, a reconstructed frame is generated to provide reference pixel data used for coding following blocks. For certain video coding standards, in-loop filter(s) may be used for enhancing the image quality of the reconstructed frame. For example, a de-blocking filter is used by a video encoder to reduce the blocky artifacts which appear after block-based coding. A video decoder is used to perform an inverse operation of a video encoding operation performed by the video encoder. Hence, the video decoder also has in-loop filter (s) used for enhancing the image quality of the reconstructed frame. For example, a de-blocking filter is also used by the video decoder to reduce the blocky artifacts.
In general, the omnidirectional video content corresponding to the sphere is transformed into a sequence of images, each of which is a projection-based frame with a 360-degree image content represented by one or more projection faces arranged in a 360-degree Virtual Reality (360 VR) projection layout, and then the sequence of the projection-based frames is encoded into a bitstream for transmission. However, the projection-based frame may have image content discontinuity at picture boundaries (i.e., layout boundaries) and/or face edges (i.e., face boundaries). A conventional de-blocking filter design, however, is unable to deal with de-blocking at discontinuous picture boundaries and/or discontinuous face edges.